Electrically powered light sources predominantly run off of the electrical grid, and are therefore powered by time-varying electrical signals, such as periodic waveforms of alternating current and voltage polarities, which are generally referred to as alternating current (AC) waveforms. The AC waveforms are generally periodic waveforms having a fundamental frequency. For example, the AC waveforms may have standard frequencies of approximately 50 Hz or approximately 60 Hz depending on the country in which and the electrical grid on which the waveforms are distributed.
The electrically powered light sources convert the electrical energy received from the electrical grid into light energy in order to provide artificial illumination. Because the electrical signal (and associated electrical energy) received by the light source from the electrical grid is time-varying, the light energy output by the light source can also be time-varying. Certain types of electrically powered light sources may thus provide lighting having a time-varying lighting intensity. The variations in lighting intensity, referred to as flicker, can have a frequency related to the standard frequency of the electrical/power signal, such as a frequency of about 50 Hz or about 60 Hz.
The amount flicker produced by a light source may be a function of the type of light source, of the frequency of the electrical/power signal, as well as of the amplitude of the electrical/power signal. For example, in situations in which the electrical excitation signal received by a light source is modulated by a dimmer switch, the flicker of the light output by the light source may increase as the amplitude of the excitation signal (and the corresponding amplitude of the lighting intensity) is reduced.
In order to reduce the flicker in the intensity of light produced by light sources powered by AC waveforms, a need exists for medium persistence light sources that reduce the amount or intensity of the flicker.